(a) Field of the Invention
The present invention relates to a semiconductor laser diode, and more particularly, to a ridge waveguide semiconductor laser diode having a high output power and an excellent laser characteristic.
(b) Description of the Related Art
GaAs based quantum well laser diodes having an InGaAs strained quantum well active layer have been intensively researched and developed for a practical use. For realizing the above object, it is desired to increase the optical output of the laser diode. For instance, when a laser diode is used at a lasing wavelength of 0.98 xcexcm as an exciting light source for an erbium-doped fiber for use in a 1.5 xcexcm band optical fiber amplifier, or when a laser diode is used at a lasing wavelength of 1.02 xcexcm as an exciting light source for a praseodymium-doped fiber for use in a 1.3 xcexcm band optical fiber amplifier, the laser diode is required to have a high output power of about 200 mW.
Referring to FIG. 1, a typical GaAs based quantum well laser diode will be described as an example. A layer structure 10 of a conventional ridge waveguide GaAs laser diode includes, for instance, a buffer layer 14 having GaAs or AlGaAs based compound semiconductor layer, a lower cladding layer 16, a first light confinement layer 18, a second light confinement layer 20, a first strained quantum well layer 22, a barrier layer 24, a second strained quantum well layer 26, a third light confinement layer 28, a fourth light confinement layer 30, a first upper cladding layer 32, an etch stop layer 34, a second upper cladding layer 36, and a cap layer 38 consecutively formed on a n-GaAs substrate 12. The second upper cladding layer 36 and the cap layer 38 overlying the etch stop layer 34 have a mesa structure.
The chief factor for restricting the increase of the optical output from the GaAs based quantum well laser diode is catastrophic optical damage (COD) which signifies that the optical facet of the laser diode is damaged instantaneously. When the optical output increases to reach a specific value, the COD which is inherent in the GaAs based laser diode is generated to stop the function of the laser diode at the instant.
For preventing the generation of COD failure, for instance, a conventional wide mesa structure of about 4 xcexcm is adopted to decrease the optical density in the active layers; however, in case of the wide mesa structure, a so-called spatial-hole-burning wherein the optical gain of the laser diode is uneven in the direction parallel to the axis of the active layers arises to thereby tend to generate a beam steering phenomenon.
The beam steering phenomenon is known in the art and means that the light beam moves in the direction parallel to the active layers, which causes the characteristic of the optical output with respect to injected current in the semiconductor laser diode to be non-linear to have a kink in the characteristic, thereby deteriorating the laser characteristic significantly. The non-linearity means that the external differentiated quantum efficiency xe2x80x9cxcex7xe2x80x9d (=dL/d(Ixe2x88x92ITH) does not remain as a constant, wherein xe2x80x9cLxe2x80x9d represents the optical output, xe2x80x9cIxe2x80x9d represents the injected current at the optical output of xe2x80x9cLxe2x80x9d, and xe2x80x9cITH xe2x80x9d represents a threshold current. In a more extreme case, xe2x80x9cxcex7xe2x80x9d sometimes becomes almost 0. Accordingly, signal conversion cannot be effected from the injected current due to the deteriorated laser characteristic. This is particularly critical when the laser diode is coupled to an optical fiber.
For preventing the generation of the problem beam steering phenomenon in a high output power range, it is necessary to consider complicated and various effects such as the waveguide mode control of the active layers and thus, the problem is not solved in the conventional GaAs quantum well laser diode.
It is therefore an object of the present invention to provide a ridge waveguide semiconductor laser diode having a linear characteristic between the optical output and the injected current in a high optical output range.
The present invention provides a ridge waveguide semiconductor laser diode including a semiconductor substrate, at least one active layer overlying the semiconductor substrate, and at least one cladding layer having a ridge structure part and an underlying remaining part overlying substantially the entire surface of the active layer.
The remaining part has a thickness xe2x80x9cDxe2x80x9d defined by:
Dxe2x89xa7Wxc3x970.5,
wherein xe2x80x9cWxe2x80x9d is a width of a spot size having a strength of 1/e2 for a near field pattern in the active layer in a direction perpendicular to the active layer, where xe2x80x9cexe2x80x9d represents the base of the natural logarithm.
In accordance with the semiconductor laser diode of the present invention, due to the value for xe2x80x9cDxe2x80x9d, the characteristic of the optical output with respect to injected current caused by a beam steering phenomenon can be improved in terms of the kink, as a result of which an excellent linear characteristic can be obtained in a higher output power.